An olefin resin sheet is excellent in heat resistance, oil resistance and the like and has been used for forming (vacuum forming, pressure forming, etc.) to produce formings, such as various containers, cups, trays, etc.
However, having sharper melt properties than other resins such as polystyrene and polyvinyl chloride, an olefin resin sheet considerably sags when preheated for thermoforming, such as vacuum forming, so that the resulting formings are apt to suffer from forming defects, such as wrinkles, thickness variation, or holes.
It is known that the sag of an olefin resin sheet in thermoforming can be reduced by using a polyblend of a polypropylene resin and a polyethylene resin, which is commonly practiced (see JP-A-52-136247 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-55-108433 and JP-B-63-30951 (the term "JP-B" as used herein means an "examined Japanese patent publication")); adding bismaleimide to a polyolefin (see JP-A-3-52493); using a composition comprising polypropylene, an inorganic filler, and a maleic anhydride-modified or silane-modified polyolefin (see JP-A-51-69553 and JP-A-52-15542); or adding fibrous polytetrafluoroethylene to a polyolefin (see JP-A-8-165358).
In recent years the proportion of a recycled material in a molding material tends to increase, climbing to about 30 to 50% by weight in a polyolefin molding material. An olefin resin sheet molded from such a molding material with a width as large as about 900 mm or more is liable to sag particularly appreciably. In particular where a sheet is formed into thermoformings of prescribed shape with high precision, for example, a container with a cover fitted on that is designed to improve hygiene or handling properties, it must be heated for at least double the usual time, which further increases the tendency to sag.
Under such circumstances, the above-mentioned conventional techniques consisting of blending polyethylene, adding bismaleimide or mixing an inorganic filler and a modified polyolefin do not achieve sufficient improvement in sag resistance enough to be effective on wide sheets. The olefin resin sheet containing fibrous polytetrafluoroethylene is, while transparent, inferior in rigidity, heat resistance or shrink resistance and is of limited use for production of high-capacity containers or containers of specified size.
Further, it has been difficult to obtain sheeting of desired width from a polyolefin resin containing polytetrafluoroethylene by T-die extrusion because the resin has poor flow to both ends of the T-die to cause starvation.
With the recent increase of frozen food or fast food for microwave oven, vacuum forming containers having little residual strain, such as containers with a cover fitted on and deep drawn containers, have been widespread. Accordingly, it has keenly been demanded by container manufacturers to develop a forming sheet stock of larger width than 1 m to improve productivity in forming such as vacuum forming.